Marlborough Sounds salmon tested positive for a new fish disease, according to a government report made public by the Ombudsman.

After high numbers of fish died at the Waihinau Bay salmon farm in March 2012, the Ministry for Primary Industries investigated to see if disease had caused the deaths and requested fish samples from New Zealand King Salmon. These samples were tested in three laboratories, two of which were overseas.

The Kenepuru and Central Sounds Residents' Association requested a copy of the report, but were supplied one with many paragraphs blacked out.

The Ombudsman last week required the ministry to provide a copy of the full report to the association.

The ministry report's previously blacked out parts show that one laboratory found positive results for infectious salmon anemia virus, but two other laboratories tested the same fish and got negative results. No other lab has been able to find evidence of the disease in the fish.

Tests by the ministry's animal health laboratory found heart abnormalities in some of the fish but the report said the changes were general and did not point to any specific cause.

However, it did say it could be "potentially indicative" of the presence of a virus.

After a story in the Marlborough Express, trading partners asked the ministry about infectious salmon anemia virus, and samples were sent to two World Animal Health Organisation-accredited laboratories, one in Canada and one in Norway.

The Canadian lab reported positive results from more than one fish using different tests.

The Norwegian lab did not have any positive results, and neither they nor the ministry lab were able to find any virus particles in the fish samples.

The ministry report said the Canadian lab had been asked to supply supporting data for its results but had not done so.

The Ombudsman noted in his report making the ministry report public that the Canadian lab was no longer accredited to test for infectious salmon anemia virus.

New Zealand King Salmon chief executive Grant Rosewarne said it was clear the positive test was a rogue one.

Rather, he said, the problem turned out to be with the feed the salmon were fed. King Salmon had trialled different feed and found that, combined with environmental factors, caused the deaths.

"It confirms it was a rogue result."

The feed the fish were given was too high in carbohydrates, he said, and when the water reached 15.5 degrees Celsius, the fish went off their food and developed malnutrition.

Rosewarne said it wasn't the food alone that caused the deaths, but it was the food combined with water temperature.

Association spokesman Andrew Caddie said the Ombudsman's decision was pleasing, despite it taking two years to get the ministry report.

The Ombudsman firmly dismissed the ministry's contention that withholding the information was necessary to avoid prejudice to the substantial economic interests of New Zealand and he firmly endorsed the rights of citizens in a transparent, participatory democracy to have access to official information in order to participate effectively in the making and administration of laws and policies, Caddie said.